Industries such as natural gas, oil, thermoelectric power plants, chemistry, pharmaceutics, medicine and other industries face many situations when the following activities are critical: detection of the presence of one or more analytes; and quantitative measurement of one or more analytes. Space exploration requires also accurate and reliable instrumentation for measuring the concentration of various analytes such as methane (CH4), water (H2O), carbon monoxide (CO) and other analytes. There are several known methods for detecting analytes, including those discussed herein.
Some known methods of detection and quantitative measurement of one or more analytes involve resonant absorption by the analyte of a very narrow band laser beam. Such methods are preferred over other methods due to the high selectivity, sensitivity, accuracy and reliability that such methods achieve. For example, tunable diode laser absorption spectroscopy (“TDLAS”) is used extensively and is spreading progressively in spectroscopic analytical instrumentation (see: C. R. Webster, S. P. Sander, R. Beer, R. D. May, R. G. Knollenberg, D. M. Hunten, J. B.; “Tunable diode laser IR spectrometer for in situ measurements of the gas phase composition and particle size distribution of Titan's atmosphere”, Appl. Opt., 29, 7, (1990), pp. 907-917). In methods of TDLAS, the narrow band output beam generated by a tunable distributed feedback Bragg grating (“DFB”) laser is scanned across a spectral interval containing the preferred absorption line of the analyte. The absorption detection within the scanning interval indicates the existence of the analyte. The amount of absorption is dependent on the analyte concentration within the measuring volume.
One known method of TDLAS is harmonic spectroscopy, whereby the bias current of the DFB laser is modulated simultaneously with small amplitude, high frequency sine wave signal with frequency f, overlapped on low frequency sawtooth signal (see: Silver J. A; “Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods”; Appl. Opt. 31, 6, pp. 707-717; U.S. Pat. No. 7,339,168 issued 2008 Mar. 4 to Spectrasensors, Inc.; U.S. Pat. No. 6,657,198 issued 2003 Dec. 2 to Spectrasensors, Inc.; U.S. Pat. No. 7,132,661 issued 2006 Nov. 7 to Spectrasensors, Inc.; U.S. Pat. No. 8,547,554 issued 2013 Oct. 1 to General Electric Company). This method is known as 2f harmonic wavelength modulated spectroscopy (“WMS-2f”). The second harmonic (2f) component of the modulated laser beam has a peak coincident with the absorption peak of the analyte and has also two adjacent dips. The absorption by the analyte is proportional with the difference in amplitude between the peak and one adjacent dip, which is a floating reference. The analyte concentration is a function of this difference. The function coefficients are defined during calibration.
Harmonic spectroscopy has several disadvantages, including the following: (i) it involves a floating reference that introduces measurement uncertainty at low analyte concentration; (ii) widening the absorption linewidth during signal processing as is involved in harmonic spectroscopy results in overlapping narrowly spaced peaks; (iii) it does not offer any possibility for measuring the baseline, or limiting the detection of low analyte concentrations; and (iv) it does not offer any means of minimizing the influence of inherent laser power changes during the wavelength tuning by direct measurement. U.S. Pat. No. 7,586,094 issued 2009 Sep. 8 to Spectrasensors, Inc., claims baseline computation by extrapolation of measured absorption values beyond the two sides of the absorption peak. WMS-2f has non-linear changes with temperature, pressure, coexisting gas components and the like (see: U.S. Patent Application Publication No. 2013/0135619 filed 2012 Nov. 28 naming assignee Yokogawa Electric Corporation). The minimum detectable analyte concentrations are reported in the range of 10 ppb (see: Silver J. A; “Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods”; Appl. Opt. 31, 6, pp. 707-717).
Another known method is the spectrum area method, which considers the analyte concentration function of the area delimited by the shape of the absorption line of the analyte (see: U.S. Patent Application Publication No. 2013/0135619 filed 2012 Nov. 28 naming assignee Yokogawa Electric Corporation; U.S. Pat. No. 8,482,735 issued 2013 Jul. 9 to Yokogawa Electric Corporation; U.S. Patent Application Publication No. 2013/0021612 filed 2012 Jul. 20 naming assignee Yokogawa Electric Corporation). According to the inventions disclosed in U.S. Pat. No. 8,482,735 and U.S. Patent Application Publication Nos 2013/0135619 and 2013/0221612, the spectrum area changes linearly with the pressure changes and does not depend on temperature and on coexisting gases. A calibration is required for finding the dependence of analyte concentration on the area of the absorption line.
One disadvantage of the spectrum area method is that the overlap of closely spaced absorption lines causes the absorption lines to be either difficult or impossible to separate. U.S. Patent Application Publication No. 2013/0135619 (filed 2012 Nov. 28 naming assignee Yokogawa Electric Corporation) teaches that the separation of the absorption lines with strong overlapping between the spectrum areas is not possible. This patent application also describes a method for computing the spectrum area by defining the bottom part of the absorption line toward the noise region. One embodiment of the invention disclosed in this patent application uses a reference light for normalizing the intensities at the input and the output of the gas cell, making the measurements insensitive to the changes of the laser output power. Yet another embodiment of the invention described in this patent application has a sealed reference cell containing the analytes used as reference for spectrum areas. The measured spectra areas are compared to spectra areas of the analytes inside the reference cell. Thus, the spectrum area method introduces significant complications in data processing.
There are several additional disadvantages of the spectrum area method including the following: (i) the analyte concentration is related to the area of the absorption line, rather than being related to the peak value of the absorption line after subtracting the noise; (ii) the same absorption peak value can have different spectrum areas, leading to a wrong absorption value; and (iii) absorption line widening causes overlapping of narrowly spaced absorption peaks.
Another known method is the coherent ring-down spectroscopy (“CRDS”), which is based on measuring the decay rate of the power at the output of an optical ring cavity containing the analyte when a pulsed laser beam is incident into the cavity (see: Picarro, “G2401 CRDS Analyzer CO2, CO, CH4, H2O”; https://picarro.app.box.com/shared/3ncm4atiot; U.S. Pat. No. 5,528,040 issued 1996 Jun. 18 to Trustees Of Princeton University; U.S. Pat. No. 7,646,485 issued 2010 Jan. 12 to Picarro, Inc.; U.S. Pat. No. 8,537,362 issued 2013 Sep. 17 to Picarro, Inc.). CRDS is a two-step process. The initial build-up step involves a laser pulse being sent to the cavity, where it is reflected multiple times. The number of reflections depends on the quality factor of the cavity. The subsequent ring-down step involves the laser beam being turned-off. If the laser wavelength is not coincident with an absorption line of an analyte inside cavity, the decay time is very short. If there is a resonant absorption inside the cavity by the analyte, the decay time is proportional with the analyte concentration.
The known methods do not achieve measurement accuracy for the detection of the presence of one or more analytes, and quantitative measurement of one or more analytes. What is needed is an invention that is operable to achieve such measurement accuracy.